usb挂起与文件描述符

问题分析

当 USB 设备挂起时，已经打开的文件描述符通常不会自动关闭，这是因为：

1. 文件描述符是内核资源：即使底层硬件不可用，文件描述符仍存在于进程中

2. 引用计数机制：只要进程持有引用，描述符就不会自动释放

3. USB 挂起 ≠ 设备移除：挂起是电源管理状态，设备仍在系统总线上

可能的原因

1. 描述符状态变化

// 典型的现象
int fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY);
// 挂起 USB 设备
system("echo suspend > /sys/bus/usb/devices/X-X/power/level");
// fd 仍然有效，但 I/O 操作会失败或阻塞

2. 检查描述符状态的几种方法

# 查看文件描述符信息
ls -l /proc/<PID>/fd/
ls -l /proc/<PID>/fdinfo/# 查看 USB 设备状态
lsusb -t
cat /sys/bus/usb/devices/X-X/power/runtime_status

3. 程序示例：检测挂起状态

c

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/ioctl.h>
#include <linux/serial.h>int check_port_status(int fd) {struct serial_struct serinfo;if (ioctl(fd, TIOCGSERIAL, &serinfo) < 0) {perror("TIOCGSERIAL failed");return -1;}// 检查端口状态printf("Port type: %d\n", serinfo.type);printf("Port flags: 0x%x\n", serinfo.flags);return 0;
}int main() {int fd = open("/dev/ttyUSB0", O_RDWR | O_NOCTTY);if (fd < 0) {perror("open failed");return 1;}printf("Descriptor opened: fd=%d\n", fd);// 尝试读取（可能会阻塞或失败）char buf[10];int n = read(fd, buf, sizeof(buf));if (n < 0) {printf("Read failed: %s (errno=%d)\n", strerror(errno), errno);}close(fd);return 0;
}

解决方案

方案1：监控 USB 事件并重新初始化

c

// 使用 udev 监控 USB 事件
#include <libudev.h>
#include <poll.h>void monitor_usb_events() {struct udev *udev = udev_new();struct udev_monitor *mon = udev_monitor_new_from_netlink(udev, "udev");udev_monitor_filter_add_match_subsystem_devtype(mon, "usb", NULL);udev_monitor_enable_receiving(mon);int fd = udev_monitor_get_fd(mon);struct pollfd pfd = {.fd = fd, .events = POLLIN};while (1) {poll(&pfd, 1, -1);if (pfd.revents & POLLIN) {struct udev_device *dev = udev_monitor_receive_device(mon);const char *action = udev_device_get_action(dev);const char *syspath = udev_device_get_syspath(dev);printf("USB event: %s on %s\n", action, syspath);if (strcmp(action, "suspend") == 0) {// 处理挂起事件handle_suspend_event();} else if (strcmp(action, "resume") == 0) {// 处理恢复事件handle_resume_event();}udev_device_unref(dev);}}
}

方案2：使用非阻塞模式和超时

c

// 设置非阻塞 I/O 和超时
int setup_serial_timeout(int fd) {// 设置非阻塞int flags = fcntl(fd, F_GETFL, 0);fcntl(fd, F_SETFL, flags | O_NONBLOCK);// 设置超时struct termios options;tcgetattr(fd, &options);options.c_cc[VMIN] = 0;      // 最小字符数options.c_cc[VTIME] = 10;    // 超时（1秒 = 10*0.1秒）tcsetattr(fd, TCSANOW, &options);return 0;
}

方案3：完整的重连机制

c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <sys/epoll.h>#define MAX_EVENTS 10typedef struct {int fd;char device[64];int connected;
} serial_port_t;void reconnect_port(serial_port_t *port) {if (port->fd >= 0) {close(port->fd);port->fd = -1;}int retries = 5;while (retries-- > 0) {port->fd = open(port->device, O_RDWR | O_NOCTTY | O_NONBLOCK);if (port->fd >= 0) {printf("Reconnected to %s\n", port->device);port->connected = 1;// 重新配置串口参数struct termios options;tcgetattr(port->fd, &options);cfsetispeed(&options, B9600);cfsetospeed(&options, B9600);tcsetattr(port->fd, TCSANOW, &options);break;}sleep(1);}if (port->fd < 0) {fprintf(stderr, "Failed to reconnect to %s\n", port->device);port->connected = 0;}
}int main() {serial_port_t port = {.fd = -1,.device = "/dev/ttyUSB0",.connected = 0};// 初始连接reconnect_port(&port);int epoll_fd = epoll_create1(0);struct epoll_event ev, events[MAX_EVENTS];ev.events = EPOLLIN | EPOLLERR | EPOLLHUP;ev.data.fd = port.fd;epoll_ctl(epoll_fd, EPOLL_CTL_ADD, port.fd, &ev);while (1) {int nfds = epoll_wait(epoll_fd, events, MAX_EVENTS, 1000);for (int i = 0; i < nfds; i++) {if (events[i].events & EPOLLHUP) {printf("Device disconnected or suspended\n");port.connected = 0;reconnect_port(&port);}if (events[i].events & EPOLLIN) {char buffer[256];ssize_t n = read(port.fd, buffer, sizeof(buffer));if (n > 0) {// 处理数据} else if (n < 0 && errno != EAGAIN) {// 错误处理port.connected = 0;reconnect_port(&port);}}}}return 0;
}

建议的调试步骤

1. 确认挂起状态

   bash

   cat /sys/bus/usb/devices/<bus>-<port>/power/runtime_status

2. 监控内核消息

   bash

   dmesg -w
   # 或
   journalctl -f -k

3. 测试文件描述符

   bash

   # 检查文件描述符是否还有效
   ls -l /proc/self/fd/3
   stat /proc/self/fd/3

4. 使用 strace 跟踪

   bash

   strace -e openat,close,read,write -p <PID>

最佳实践

1. 总是检查 I/O 操作的返回值

2. 实现超时和重试机制

3. 使用非阻塞模式或 select/poll/epoll

4. 监控 udev 事件来响应硬件状态变化

5. 实现优雅的重连逻辑

文件描述符在 USB 挂起时不会自动消失是正常行为，你需要主动检测和处理挂起/恢复事件，并实现相应的错误恢复机制。